1. Field of the Invention
The present invention relates to a funnel geometry of a mold for the continuous casting of metal. The mold has a pouring portion with cooled long side walls and short side walls, wherein the pouring portion becomes narrower in the shape of a funnel in the casting direction until it reaches the size of the cast strand.
2. Description of the Related Art
The dimensions of the pouring portion are essentially determined by the cross-section of the strand to be cast, the dimensions of the pouring pipe and the depth of immersion of the pouring pipe in the molten metal.
Because of the funnel-shaped configuration of the long side walls, the mold not only narrows in the casting direction, but also a change of the shape of the strand cross-section takes place. Consequently, in contrast to a conventional continuous casting mold with planar walls, additional deformations are imparted on the strand shell when it travels through a funnel-type mold. These deformations may lead to defects.
It is known in the art that, by selecting the configuration of the horizontal or vertical contours of the funnel-shaped pouring portion of a continuous casting mold, the deformations imparted to the strand can be distributed more favorably in order to counteract any generation of defects of the cast product.
In contrast to a conventional mold with planar or flat walls, in which the shrinkage of the strand is compensated by the inclination of the short side walls, in a continuous casting mold with a funnel-shaped pouring portion it is of great significance how the mold cross-section narrows in the casting direction.
If in one or more vertical sections of the mold the cross-section of the mold narrows to a greater extent than the shrinkage of the strand, additional deformations are imparted to the strand shell. Also, in that case, a uniform contact between the strand shell and the mold wall is no longer ensured. Areas with excessively high or low strand shell temperatures are created, which increases the probability of the generation of defects.
If the mold does not narrow enough, the strand shell will partially loose contact with the mold wall. The substantial reheating in these areas produces high thermal stresses which lead to defects.
The defects referred to are cracks, constrictions and disturbances of the structure. These defects become more serious the more the steel has the tendency to comparatively high shrinkage during the solidification and cooling of the strand shell being formed.
EP 0 268 910 B1 proposes to guide the strand shell which is still thin underneath the meniscus without deformation by providing long side walls which in the pouring portion extend essentially parallel to each other in a first section and are reduced to the thickness of the casting size in a subsequent section, wherein the first section extends to below the meniscus level to be adjusted during the casting operation into the area of the first strand shell formation. In this manner, the strand shell which is still thin underneath the meniscus is to be guided free of deformations.
EP 0 552 501 A2 discloses a mold for the continuous casting of steel strip in which the long side walls form a funnel-shaped pouring portion which is reduced toward the short side walls and in the casting direction to the size of the cast strip. The funnel-shaped pouring portion is determined by lateral circular arcs and middle circular arcs connected to the lateral circular arcs at the tangential points. For reducing the friction and wear and for reducing the tensile stress and bending stress of the strand shell, the radii of the lateral circular arcs are uniform in a section which extends at least 100 mm down from the upper edge of the mold.
DE 39 07 351 A1 discloses a proposal for forming the contour of the inner wall of the pouring funnel in the strand travel direction by three circular arcs which contact each other tangentially, wherein the radii of the circular arcs gradually increase in the strand travel direction and lead into the contour of the inner wall of the mold. In this manner, the deformation of the metal cast strand is distributed over a travel length which is as long as possible and constrictions and the formation of cracks in the strand shell of the metal cast strand are avoided. A distribution of the change of the shape of the strand shell which is as uniform as possible in a pouring portion shaped in this manner is achieved thereby that the radii of the circular arcs increase in the strand travel direction with equal or unequal factors.
Therefore, it is the primary object of the present invention to provide a further funnel geometry of a mold which improves the reduction of friction and wear between the strand shell and the mold walls. Particularly in the case of a strand shell formation of a steel which has the tendency to be subject to relatively high shrinkage, an even more pronounced reduction of the defects formed at the slab surface is to be realized by a narrowing of the mold cross-section which is adapted to the shrinkage behavior of the steel.
The present invention is based on the finding that, with a given horizontal contour of the funnel portion, the configuration or course of the narrowing of the mold cross-section in the casting direction can be predetermined by the shape of the vertical contour.
In accordance with the present invention, in the funnel portion the inner contour of the long side walls is constructed convexly along a straight line connecting the upper edge of the mold and the outlet of the pouring portion.
The convex inner contour may have a constant curvature or a changing curvature.
In accordance with a further development of the invention, the convex inner contour of the long side walls has a configuration which is circular arc-shaped, polygonal or trigonometrical.
In accordance with another development of the invention, the convex inner contour of the pouring portion extends to the outlet of the mold.
When the lower section of the mold has a portion with parallel walls, a particularly friction-free transition between the funnel portion and the subsequent portion with parallel walls is achieved thereby that the convex contour leads with a lower circular arc at the end into the straight portion having a steady configuration.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.